In the automotive industry, formed plastic parts are often painted. This is to provide a smooth, oftentimes metal-like finish. The paint covers molding and other substrate imperfections. Decorative effects such as mirror finishes, texturing and color matching of differently processed parts are also achieved by painting.
Examples of plastics that are paintable and used in the automotive industry include polycarbonate/polyethyleneterephthalate, polycarbonate/acrylonitrile-butadiene-styrene, polycarbonate/polybutyleneterephthalate, polyphenylene oxide/nylon 66, polyphenylene oxide polybutyleneterephthalate, poly-carbonate (both untilled and glass-filled), polyphenylene oxide (both filled and untilled), resin/glass material, and thermoplastic olefins. The use of plastics in the automotive area and other areas is growing due to its durability, lightweight, and low cost. Plastics need to be cleaned before painting them. The most common contaminants that must be removed before painting include mold release agents, shop soil, material handling soil, machining soil, fingerprints and plastic sanding dust. These contaminants are removed by chemical means such as detergents.
The process of cleaning plastic parts generally entails a wash stage, a rinse stage and a final rinse stage with deionized water. The plastic parts can be either spray washed or immersed in the detergent. The most common processes are either an acidic detergent wash stage followed by multiple rinse stages, or an alkaline detergent stage followed by a rinse stage, an acidic detergent wash stage, and multiple rinse stages. For example, one process employs in stage one an alkaline wash, followed by a tap water rinse in stage two. Stage three is an acidic wash with a tap water rinse at stage four. Stages five and six are deionized water rinses.
When the plastic part is to be painted, the cleaning operation is performed in a production line with freshly cleaned parts subsequently conveyed into the paint booth. Problems can occur however, when the freshly cleaned part is not fully dry. This can lead to "spotting" when painted and will result in an imperfect paint job. This will require stripping of the part and/or additional painting procedures.
Further, when a bead of water dries on the plastic surface, ionic salts such as Cl, Ca, Mg, and K can remain on the surface. After the plastic has been painted, there will be a visual defect caused by such deposits. Exposure to humidity which can cause increased water sensitivity can lead to blistering of the paint. Adhesion problems could also result.
Removing the aqueous rinse water from the surface of the plastic requires a drainage enhancing solution which can effectively aid in drying the plastic part to be painted. The present inventors have discovered an aqueous solution which will aid in reducing spotting caused by poor drainage on the external surfaces of plastic parts. More surprisingly, the present inventors have discovered a drainage enhancing solution that is not rinsed off the surfaces of plastic parts by subsequent rinse stages. The use of rinse aids in the cleaning of plastic parts is known. For example, U.S. Pat. No. 5,314,546 discloses a process for enhancing the drainage of residual aqueous rinse from the external surfaces of plastic parts comprising a zircoaluminate compound.